InnoDB tables use row-level locking so that
multiple sessions and applications can read from and write to
the same table simultaneously, without making each other wait or
producing inconsistent results. For this storage engine, avoid
LOCK TABLES statement,
because it does not offer any extra protection, but instead
reduces concurrency. The automatic row-level locking makes these
tables suitable for your busiest databases with your most
important data, while also simplifying application logic since
you do not need to lock and unlock tables. Consequently, the
InnoDB storage engine is the default in MySQL
MySQL uses table locking (instead of page, row, or column
locking) for all storage engines except
InnoDB. The locking operations themselves do
not have much overhead. But because only one session can write
to a table at any one time, for best performance with these
other storage engines, use them primarily for tables that are
queried often and rarely inserted into or updated.
When choosing whether to create a table using
InnoDB or a different storage engine, keep in
mind the following disadvantages of table locking:
Table locking enables many sessions to read from a table at the same time, but if a session wants to write to a table, it must first get exclusive access, meaning it might have to wait for other sessions to finish with the table first. During the update, all other sessions that want to access this particular table must wait until the update is done.
Table locking causes problems when a session is waiting because the disk is full and free space needs to become available before the session can proceed. In this case, all sessions that want to access the problem table are also put in a waiting state until more disk space is made available.
SELECT statement that takes
a long time to run prevents other sessions from updating the
table in the meantime, making the other sessions appear slow
or unresponsive. While a session is waiting to get exclusive
access to the table for updates, other sessions that issue
SELECT statements will queue
up behind it, reducing concurrency even for read-only
The following items describe some ways to avoid or reduce contention caused by table locking:
Consider switching the table to the
InnoDB storage engine, either using
CREATE TABLE ... ENGINE=INNODB during
setup, or using
ALTER TABLE ...
ENGINE=INNODB for an existing table. See
Chapter 14, The InnoDB Storage Engine for more details
about this storage engine.
SELECT statements to
run faster so that they lock tables for a shorter time. You
might have to create some summary tables to do this.
Start mysqld with
storage engines that use only table-level locking (such as
MERGE), this gives all statements that
update (modify) a table lower priority than
SELECT statements. In this
case, the second
statement in the preceding scenario would execute before the
UPDATE statement, and would
not wait for the first
To specify that all updates issued in a specific connection
should be done with low priority, set the
system variable equal to 1.
Start mysqld with a low value for the
variable to force MySQL to temporarily elevate the priority
SELECT statements that
are waiting for a table after a specific number of inserts
to the table occur. This permits
locks after a certain number of
Splitting table contents into separate tables may help, by allowing queries to run against columns in one table, while updates are confined to columns in a different table.
You could change the locking code in
mysys/thr_lock.c to use a single queue.
In this case, write locks and read locks would have the same
priority, which might help some applications.